Reproduction of sound



@c&. so, 1945. R COONEY 2,387,832

REPRODUCTION OF SOUND Filed April 18, 1944 Z I, FMPL/F/EE Snventor Jzm A? Lmm y (Ittorneg Patented Oct. 30, .1945

UNITED STATES PATENT OFFICE REPRODUCTION 01? soumi m It. Cooney, Waldoboro, Maine Application April s, 1944, Serial No. 531,560

' 7 Claims. -(01. ire-100.3)

This invention relates to the reproduction of sound from sound records which tend to produce undesirable background noise due to lack of homogeneity or other imperfections in the record medium. It has for its principal object the provision of an improved apparatus and method of operation whereby such background noise may be minimized or obviated during the reproduction of sound from the record.

The record itself may be a narrow transparent line which is produced by a galvanometer or the like energized by the recorded audio frequency current. Such a line is commonly known as a sound record of the snake type. In such a rec- 6rd, the position of the transparent line at a y point transversely of the record is determined by the polarity and amplitude of the recorded audio impulses.

Various means for reproducing sound from records of the snake track type have been proposed.

The present invention difiersfrom these prior proposals in that it provides means whereby the electrical impulses tending to produce background noise are obviated in the sound reproducer and do not appear'in the output delivered to the loudspeaker or other utilization device.

A photocell can recover modulation from a snake track by virtue of a light wedge which allows it to discriminate between the various positions occupied by the trace, and to produce a different output voltage for every different position of the trace. However, because the width of the snake track remains constant, the actual amount of light passed by the film is always constant except insofar as affected byvariatlons in transmission of the clear area of the film. These variations produce "noise in the photocell output; It is apparent that if every time the transis merely to maintain a constant illumination on the photocell side of the track; i. e., when the transmission of the sound track decreases the monitoring photocell current decreases and the decrease is amplified by the feedback amplifier and applied to the light source in such phase that the light increases, and vice versa. If the amplifier has a large gain the light received by versa, compensation of these variations in transmission is realized, and the light passed by the sound track appears to be constant. However, because of the wedge, the photocell still produces different outputs for difiereht positions of the sound track.

Thus if there is provided an auniliary photocell which is independent of thesound reproducing photocell and is arranged: so as to observe the snake track directly, the output of this photocell may be amplified in an entirely separate amplifier and returned so as to modulate the light source in a degenerative sense. The efiect of this auxiliary regulating or monitoring system the auxiliary photocell will thus be maintained practically constant, regardless of variationsin transmission of the film. Motion from side to side produces no effect on this monitoring photoelectric system, but does on the sound reproducin photocell which is illuminated through a wedge and appears to observe a perfectly clear, steadily illuminated sound track, which weaves from side to side and produces sound by virtue of its position alone. a

" The compensating voltage produced by the auxiliary photocell and degenerative amplifier may be applied to elements other than the exciter lamp with very much the same result.

For example, the current in a gas-filled photo cell is a function of both the incident light and the anode potential. Likewise the current in a multiplier-type photocell can be modulated by varying the potential of one or more of its dy-' nodes. Assuming that both these eflects are substantially linear, the output of either of these devices is proportional to the-product of the incident light and the potential applied to this control electrode. If no light is falling on the device, no current flows through it,.and any variation of the control potential produces no effect on the output. When a, photo-current is present, I

variation of the control potential produces a variation in the output current of the device, and the A. C. output depends on themagnitude of to reach the output circuit directly. This is possible in a gas-filled cell by varyin the anode potential and taking the output from a resistor on the cathode side, etc., and is inherent in a multiplier type tube.)

. Now if we allow one of these photo-sensitive devices to observe a fluctuating light source, am-' sible. It will be seen on consideration that the A. C. voltage atthe control electrode depends only on the percentage modulation of the incident light-i. e., this voltage remains practically constant for any given percentage modulation of the light, irrespective of the strength of the illumination. (Furthermore, whether or not the relation between control anode potential and output is linear is immaterial, as the feedback voltage will automatically be given the proper waveform to take care of this.)

If any auxiliary gas-filled photocell is illuminated through a moving snake track (without any wedge) and its output is amplified and returned to the anode as discussed above, in such a manner that the feedback potential cannot actually reach the amplifier input, but can merely modulate the anode potential, the anode potential of the photocell will now vary in such a way as to tend to maintain a constant, unvarying current through the photocell. As discussed above this A. C. anode voltage will only represent the percentage variation of the sound-track transmission. If the exciter lamp intensity should be increased, this voltage would still maintain the same wave-form and amplitude. In other words, the anode potential is varying in just the proper amount to compensate for the "noise in the sound track regardless of the average illumination level.

If the control anode of a sound reproducing photocell is connected to the anode of the above auxiliary photocell and a 'wedge in front of this sound reproducing cell transmits light to it to observe the same from the snake track, the photo-current produced by this sound reproducing cell is then dependent on the position of the snake track. However the percentage variation due to noise is unaffected by the wedge, and as the anode potential of this sound reproducing cell is varied by the monitoring system just the proper amount (assuming the two photocells are exactly similar), the output of the sound reproducing cell represents the modulation of the sound track free of the noise.

Important objects of the invention are the provision of an improved sound reproducer and an improved method of the noiseless reproduction of sound.

The invention will be better understood from the following description considered in connecscope is indicated by the appended claims.

Referring to the drawing:

Figure 1 illustrates a type of sound record suitable for use in connection with the present invention,

. Figure 2 illustrates one embodiment. of the invention, and

Figure 3 illustrates a modified form invention.

As previously indicated, the record ll of Fig. l is in the form of a narrow transparent line which has its position transversely of the record determined by the polarity and amplitude of the recorded sound. Since the width of this transparent line is constant, it is apparent that it transmits a constant amount of light solong as its transparency and intensity of illumination are kept constant. when the intensity of illumination or the transparency of the line varies, however, the amount of light transmitted by the line varies and background noise results. This background noise is minimized by the system of Fig. 2.

The sound reproducing apparatus of Fig. 2 includes an exciter lamp II and an optical unit of the I! through which a narrow line of light is applied transversely of the record II. The light modulated by the record l0 next passes through an optical unit (indicated as a lense II) and is thereafter applied to a semi-transparent reflector I l and a total reflector l5. It is apparent that means other than the reflectors l4 and I5 may be provided to separate the modulated light into two separate beams. From the reflector is, light is passed through a light wedge, a graded filter I6 or the equivalent, to a photosensitive device H from which the reproduced audio impulses are delivered through an output transformer l8.

Connected in circuit with the transformer 18 are a. potential source or battery l9 and a resistor 20. The battery l9 and resistor 20 are arranged also to be connected through a coupling capacitor 2| and a double throw switch 22 to the output circuit of a degenerative amplifier 21. Potential is applied to the input of the amplifier 21 from the cathode of an auxiliary or control photosensitive device 23 through a resistor 30. The leads 3| may be interconnected with the exciter lamp- IS in any. manner suitable for modulating the intensity of this lamp in response to variation in the output of the amplifier 21. Neutralization of the capacity of the photosensitive devices l1 and 23 may be accomplished by the provision of a neutralizing capacitor bridge circuit as is well known.

'tion with the accompanying drawing, and its' Assuming the switch 22 to be in its left hand closed position and the leads ,3! to be connected to the exciter lamp II as previously indicated, the output of the photosensitive device 23 is amplified'by the alternating current amplifier 21 and the amplified output is utilized tomodulate the intensity of the light source II in such phase as to oppose any change in the current of the device 23. Then if the gain of the dengenerative 1 feedback amplifier 21 be sufilciently large, the light received by the device 23 will be approximately constant regardless of low percentage alternations or variations in the density of the record through which light is applied to the device. The sam'e obviously is true of the photosensitive device l1.

Due to the -fact that it receives light through the wedge IS, the device I! delivers acurrent which varies with change in the position of the transparent lin'e transversely of the record and is truly representative of the sound impulses recorded on the record. Thus background noise is substantially eliminated in the absence of recorded sound and regardless of the degree to .which the recorded sound is modulated.

The same result can be achieved in other ways. If the photosensitive devices l1 and 23 be of the gas-filled type, for example, their outputs are a function of both the incident light and the anode potential, varying approximately as the product of the two.

With the switch 22 closed to the right, variation in the 'output of the photosensitive device 23 functions through the amplifier 21 to modulate the anode potential of the devices I! and 23 as explained above in connection with the modulation of the light source ll. Thus the anode potential is increased when the light passed by the transparent line tends to decrease and vice verso. Under these conditions the space current of the two cells is maintained substantially independent of variation in the transparency of the record. As before, the audio output is determined by the position of the transparent line transversely of the record.

The modification of Fig. 3 differ from that of graded filters or the equivalent -(indicated as N and 25) to diiierent parts or which the modulated beam is applied through a beam splitting device 28 which is equivalent to the reflectors I4 and I! of Fig. 2. The light filters 2d and 25 are constructed to give a transmission varying linearly along their length from zero to full transmission.

These two light filters are identical but are reversed with respect to each other so.that their transmission varies in opposite directions.

The modificationof Fig. 3 further diflers from that of Fig. 2 in that the photosensitive elements I! and 23 are connected in push-pull to provide a higher output voltage. In this case, the illumination of' the elements l1 and that of the element 23 are the same when the transparent line is on the median line of the record so that the output voltage E is zero and independent of light transmitted through the sound track. 1 As the transparent line moves toward one side of the record, the filter 28 transmits more light than .the filter 25 so that the currents of the elements I! and 28 become unbalanced in the transformer I8 and an output current representative of the displacement or position of the transparent line is developed.

The total current of the resistor 30, however, remains substantially constant and any tendency for it to change because of variation in the sound track transmission is resisted by the degenerative feed-back through the amplifier 21 to the anodes of the elements i1 and 23. The feed-back voltage thus produced across the resistor and applied to the anodes of the elements I! and 23 should not reach the transformer t8 directly since it is a modulating voltage and is eflective only insofar as there is a photo current present forit to modulate. When the filter 24 transmits inore light than the filter 25, the photosensitive element I'i is supplying most of the output current and also most of the noise but receives the benefit of the feed-back voltage in proportion so that the net result is the cancellation of noise in the output.

The sum of the currents of the photosensitive elements ii and 2Q (Fig, 3) is always constant (except for noise and the like) irrespective of the position of the snake traclc Together they act' like a single element observing the transparent line without a wedge. The sumo! their currents taken from the center tap of the transformer I 8 can thus be utilized to provide a feedback voltage. The advantage of the system of Fig. 3 over that of Fig. 2 is that the output voltage is doubled.

The outstanding feature of the invention is an improved sound reproducing apparatus wherein v the noise resulting from defects in the record medium Tare segregated from the signal impulses delivered to the loudspeaker or other utilization beam into a pair of beams, a pair of photosensitive elements each arranged to receiv light from a different one of said beams, means interposed in the path or one beam of saidlpair for producing in one oi said photosensitive elementscurrent 1 of an amplitude dependent on the position of said beams, and means responsive to the output of the other of said photosensitive elements for regulating the intensity of said beams.

2. The combination of means for producing a single light beam which varies in position along a predetermined line and has an intensity which varies only as aresult or imperfection in said beam producing means, means for separating said beam into a pair of beams,a pair-of photosensitive elements each arranged to receive light from a different one of said beams, means interposed in the path of one beam of said pair for producing in one of said photosensitive elements current of an amplitude dependent on the position of said beams, and means responsive to the output-of the other of said photosensitive elements for obviating the eifect of variation in the intensity or single light beam.

3. The combination of means for producing a single light beam which varies in position along a predetermined line and has an intensity which varies only as a result of imperfection in said beam producing means, means for separating said beam into a pair of beams, a pair 01' photosensitive elements each arranged to receive. light from a suchvariation in intensity.

4. The combination of means for producing a single light beam which varies in position along a predetermined line and has an intensity which varies only as a result of imperfection in said beam producing means, means for separating said beam into a pair of beams, apair of photosensitive elements each arranged to receive light from a different one of said beams, means interposed in the path of one beam of said pair for producing in one of said photosensitive elements current of an amplitude dependent on the position of said beams, and means including a degenerative amplifier having its .input connected to the cathode of the other of said elements and its output connected to the anodes of said elements for minimizing the effect of variation in the intensity of said single light beam.

5. The combination of means for producing a light beam which varies in position'and has an intensity which varies only as a result of imperfaction in said beam producing means, means responsive to said beam for detecting its position, and means responsive to said beam for rendering said position detecting means independent of variations in said intensity.

6. The combination oi means forproducing a single light beam which varies in positionand has an intensity varied only as a result of imperfections in said beam producing means, means for separating said single beam into a pair of beams, a pair of photosensitive elements eacharranged to receive light from a different beam of said pair, means interposed in the paths of 'the beams of said pair for producing insaid elements currents of an amplitude dependent on the position of said single beam, a push-pull transformer connected tov the cathodes or said elements, and means connected between a midpoint terminal of said transformer and the anodes of said elements for minimizing the eiiect of variacurrents of an amplitude dependent on the position of said single beam, 9. push-pull transformer connected to the cathodes of said elements, and a degenerative amplifier having its input connected to a midpoint terminal of said trans former and its output connected to the anodes of said elements for minimizing the eflect of variation in the intensity of said light beams on the currents of said elements.

* JOHN R. COONEY. 

